This invention is a circuit for controlling the number of elements in thermal print heads that are activated at one time to prevent the overloading of the power supply, and more specifically, is a circuit for limiting the number of active elements to the exact number that can be loaded onto the power supply, regardless of the spacing of the data bits to be printed.
A thermal printer operates by activating a plurality of print elements, one element for each black pixel in the line. In the worst case, all pixels in the line will be black and all heating elements will have to be on simultaneously. A power supply large enough to handle this load therefore must be provided.
One way to reduce the size of the power supply is to divide up the line to be printed into segments and print the segments one at a time. This reduces the printing speed.
To reduce the size and cost of the required power supply, while maintaining some of the printing speed of a full line printer, Haruhiko Moriguchi et al, in U.S. Pat. Nos. 4,447,819 and 4,454,516, describe a system wherein the heating elements are divided into equal groups, and the black pixels in the current line are counted. If the number of black pixels in the line does not exceed the number of pixels in one group of heating elements, then the line is printed one segment at a time. To use a numerical example, assume there are 1600 bits, or pixels, per line, and that the line is divided into four segments. Then, the number of black pixels in the current line is determined. If it is less than or equal to 400, the entire line is printed. If it is more than 400, each segment is printed separately. In this case, the power supply can be designed to handle a maximum load of 400 pixels, instead of 1600, while the printing speed is still maintained at a reasonable rate because on a typical printed page, many lines will have fewer than 25% black pixels.
An improvement in speed can be accomplished by dividing the line into a number of segments, depending on the number of black pixels in the line. For example, if there are between 25% and 50% black pixels, the line can be divided into two groups while if there are more than 50% black pixels the line must be divided into four segments. Of course, there is a problem in this case if the pixels are not evenly spaced over the line.